In recent years, a bottlenecking of communication networks has occurred in the portion of the network known as the access network. Bandwidth on longhaul optical networks has increased sharply through new technologies such as WDM and transmission of traffic at greater bit rates. Metropolitan-area networks have also seen a dramatic increase in bandwidth. However, the access network, also known as the last mile of the communications infrastructure connecting a carrier's central office to a residential or commercial customer site, has not seen as great of an increase in affordable bandwidth. The access network thus presently acts as the bottleneck of communication networks, such as the internet.
Power-splitting passive optical networks (PSPONs) offer one solution to the bottleneck issue. PSPONs refer to typical access networks in which an optical line terminal (OLT) at the carrier's central office transmits traffic over one or two downstream wavelengths for broadcast to optical network units (ONUs). In the upstream direction, ONUs typically time-share transmission of traffic in one wavelength. An ONU refers to a form of access node that converts optical signals transmitted via fiber to electrical signals that can be transmitted to individual subscribers and vice versa. PSPONs address the bottleneck issue by providing greater bandwidth at the access network than typical access networks. For example, networks such as digital subscriber line (DSL) networks that transmit traffic over copper telephone wires typically transmit at a rate between approximately 144 kilobits per second (KB/s) and 1.5 megabits per second (MB/s). Conversely, Broadband PONs (BPONs), which are example PSPONs, are currently being deployed to provide hundreds of megabits per second capacity shared by thirty-two users. Gigabit PONs (GPONs), another example of a PSPON, typically operate at speeds of up to 2.5 gigabits per second (GB/s) by using more powerful transmitters, providing even greater bandwidth. Other PSPONs include, for example, asynchronous transfer mode PONs (APONs) and gigabit Ethernet PONs (GEPONs).
Although PSPON systems have increased bandwidth in access networks, demand continues to grow for increased bandwidth. Although some cost-effective solutions have been proposed for increasing bandwidth in the downstream direction, such as wavelength-shared hybrid PONs (WS-HPONs), few cost-effective solutions have been proposed for increasing upstream capacity.
One upstream solution, wavelength division multiplexing PON (WDMPON), would increase upstream (and downstream) capacity dramatically but inefficiently. WDMPONs refer to access networks in which each ONU receives and transmits traffic over a dedicated downstream and upstream wavelength, respectively. Although WDMPONs would increase capacity dramatically, they would do so at a prohibitively high cost for many operators and would supply capacity far exceeding current or near-future demand. Because demand for greater upstream capacity continues to grow (but not at a rate to justify adoption of WDMPONs in most cases), a need exists for cost-efficient solutions along the upgrade path from PS-PONs to full WDMPONs.